Spectral Distributions of Clear Sky Light and Their Chromaticities

The spectral radiant flux distribution of clear sky light varies mainly from the following factors: aerosol density, solar altitude and angular distance from the sun, and reflected light from the ground surface. This paper investigates the relation between cause and effect of variation of clear sky light. The spectral radiant flux distributions of clear sky light are not measured but calculated theoretically. The following conclusions were obtained: (1) The chromaticities of daylight in high latitudes are distributed up to the higher region of correlated color temperature (CCT) than in low latitudes and a number of chromaticities of daylight in low latitudes are distributed at the low region of CCT compared with in high latitudes. (2) The chromaticities of north sky light in low latitudes are distributed with a slight shift into green side at high CCT and into purple side at low CCT compared with that of north sky light in high latitudes. (3) The difference of the distributions of chromaticities of daylight falling on a horizontal surface of the ground between in high latitudes and in low latitudes is much smaller than that of the distributions of chromaticities of north sky light between in high latitudes and in low latituydes.